as u can see, in the case of ++a, 1st a is incremented then its value is asigned to c, in the 2nd case. 1st, b's value is assigned to d.
There's one more place which this makes a diff(there may be more. cant think of any right now ). Suppose:

int i = 0;
while (i++<10)
{
/*statements*/
}

and

int i = 0;
while (++i<10)
{
/*statements*/
}

in the first case, i is first compared with 10 then it is incremented. In the second case, i is 1st incremented then its compared. Hope u got it
Similarly u have post and pre decrement using '--'.